Method for driving liquid crystal display device

ABSTRACT

In an image signal writing period, a first image signal is supplied to a first liquid crystal element and a first capacitor from a first signal line. In a backlight lighting period, display is performed in a light-transmitting pixel portion in response to the first image signal. In a black grayscale signal writing period, a signal for black display is supplied to a second liquid crystal element and a second capacitor from a second signal line. In a still image signal writing period, a second image signal is supplied to the first liquid crystal element, the first capacitor, the second liquid crystal element, and the second capacitor from the first signal line. In a still image signal holding period, display is performed in the reflective pixel portion in response to the second image signal.

TECHNICAL FIELD

The present invention relates to a method for driving a liquid crystaldisplay device. Alternatively, the present invention relates to a liquidcrystal display device. Further alternatively, the present inventionrelates to an electronic device including a liquid crystal displaydevice.

BACKGROUND ART

Liquid crystal display devices are widely used in large display devicessuch as television receivers and small display devices such as mobilephones. Products with higher added values are required and are beingdeveloped. In recent years, in view of increase in concern about globalenvironment and improvement in convenience of mobile equipment,development of liquid crystal display devices with low power consumptionhas attracted attention.

Non-Patent Document 1 discloses a structure of a liquid crystal displaydevice in which refresh rates differ between the case of moving imagedisplay and the case of still image display for reduction in powerconsumption of the liquid crystal display device.

Non-Patent Document 2 discloses a structure of a semi-transmissiveliquid crystal display device in which color image display performed byfield sequential driving and monochrome image display performed byturning off a backlight and using reflected light are switched forreduction in power consumption of the liquid crystal display device.

REFERENCE

-   [Non-Patent Document 1] Kazuhiko Tsuda et al., IDW'02, pp. 295-298-   [Non-Patent Document 2] Ying-hui Chen et al., IDW'09, pp. 1703-1707

DISCLOSURE OF INVENTION

According to Non-Patent Document 1, power consumption can be reduced bylowering the refresh rate in displaying a still image. However, sincepower consumption of a liquid crystal display device largely depends onlighting of a backlight, the structure of Non-Patent Document 1 has aproblem in that power consumption is not sufficiently reduced. Thestructure of Non-Patent Document 2 has a problem in that fieldsequential driving in a semi-transmissive liquid crystal display deviceleads to insufficient contrast of a display image due to lightscattering or the like in a reflective pixel portion, especially underintense outside light.

Thus, an object of an embodiment of the present invention is to suppressreduction in contrast due to light scattering or the like in areflective pixel portion and to reduce power consumption.

An embodiment of the present invention is a method for driving asemi-transmissive liquid crystal display device including a plurality ofpixels. Each of the pixels has a light-transmitting pixel portion and areflective pixel portion. The light-transmitting pixel portion includesa first pixel transistor whose first terminal is electrically connectedto a first signal line and whose gate is electrically connected to ascan line, and a first liquid crystal element and a first capacitorwhich are electrically connected to a second terminal of the first pixeltransistor. The reflective pixel portion includes a second pixeltransistor whose first terminal is electrically connected to the secondterminal of the first pixel transistor and whose gate is electricallyconnected to a first selection line, a second liquid crystal element anda second capacitor which are electrically connected to a second terminalof the second pixel transistor, and a third pixel transistor whose firstterminal is electrically connected to a second signal line, whose gateis electrically connected to a second selection line, and whose secondterminal is electrically connected to the second liquid crystal elementand the second capacitor. In the method for driving thesemi-transmissive liquid crystal display device, in a first period, thefirst pixel transistor is turned on, the second pixel transistor isturned off, the third pixel transistor is turned off, and a first imagesignal is supplied to the first liquid crystal element and the firstcapacitor from the first signal line. In a second period, display isperformed in the light-transmitting pixel portion in response to thefirst image signal supplied in the first period. In a third period, thefirst pixel transistor is turned off, the second pixel transistor isturned off, the third pixel transistor is turned on, and a signal forblack display is supplied to the second liquid crystal element and thesecond capacitor from the second signal line in the reflective pixelportion. The first to third periods are repeated so that a moving imageis displayed. In a fourth period, the first pixel transistor is turnedon, the second pixel transistor is turned on, the third pixel transistoris turned off, and a second image signal is supplied to the first liquidcrystal element, the first capacitor, the second liquid crystal element,and the second capacitor from the first signal line. In a fifth period,display is performed in the reflective pixel portion in response to thesecond image signal supplied in the fourth period. The fourth period andfifth period are repeated so that a still image is displayed.

An embodiment of the present invention may be a method for driving aliquid crystal display device, in which the first image signal suppliedin the first period is an image signal corresponding to any color of R,G, and B, and backlights which emit respective colors of R, G, and B aresequentially operated in the second period.

An embodiment of the present invention may be a method for driving aliquid crystal display device, in which the second image signal is animage signal for displaying an image at a lower grayscale level than animage of the first image signal.

An embodiment of the present invention may be a method for driving aliquid crystal display device, in which time for displaying one image inthe fourth and fifth periods is longer than time for displaying oneimage in the first to third periods.

An embodiment of the present invention may be a method for driving aliquid crystal display device, in which supply of a driver circuitcontrol signal for driving the scan line and the first signal line isstopped in the fifth period.

According to an embodiment of the present invention, reduction incontrast due to light scattering or the like in a reflective pixelportion can be suppressed and power consumption can be reduced withoutmaking the structure complicated, for example, increase in the number ofdriver circuits, wirings, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram illustrating a liquid crystal display device of anembodiment of the present invention;

FIG. 2 is a diagram illustrating a liquid crystal display device of anembodiment of the present invention;

FIG. 3 is a chart showing operation of a liquid crystal display deviceof an embodiment of the present invention;

FIGS. 4A and 4B are charts showing operation of a liquid crystal displaydevice of an embodiment of the present invention;

FIGS. 5A to 5E are diagrams illustrating a liquid crystal display deviceof an embodiment of the present invention;

FIG. 6 is a diagram illustrating a liquid crystal display device of anembodiment of the present invention;

FIGS. 7A and 7B are diagrams illustrating a liquid crystal displaydevice of an embodiment of the present invention;

FIG. 8 is a diagram illustrating a liquid crystal display device of anembodiment of the present invention; and

FIGS. 9A and 9B are diagrams illustrating an electronic device of anembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention can beimplemented in many different modes, and it is easily understood bythose skilled in the art that modes and details of the present inventioncan be modified in various ways without departing from the spirit andthe scope of the present invention. Therefore, the present invention isnot construed as being limited to the description of the embodiments.Note that in structures of the present invention described below,reference numerals denoting the same portions are used in common indifferent drawings.

Note that the size, the thickness of a layer, the waveform of a signal,and a region of components illustrated in the drawings and the like inthe embodiments are exaggerated for simplicity in some cases. Therefore,the embodiments of the present invention are not limited to such scales.

Note that in this specification, terms such as “first”, “second”,“third”, and “N-th” (N is a natural number) are used in order to avoidconfusion among components and do not limit the components numerically.

Embodiment 1

In this embodiment, a method for driving a liquid crystal display devicewill be described with reference to circuit diagrams of a pixel of theliquid crystal display device, timing charts showing operation thereof,and the like.

First, a configuration will be described with reference to FIG. 1 whichis a circuit diagram of a pixel. FIG. 1 illustrates a pixel 100, a scanline 101 (also referred to as a gate line), a first signal line 102(also referred to as a data line), a second signal line 121, a firstselection line 103, and a second selection line 122. The pixel 100 has alight-transmitting pixel portion 104 and a reflective pixel portion 105.The light-transmitting pixel portion 104 includes a first pixeltransistor 106, a first liquid crystal element 107, and a firstcapacitor 108. The reflective pixel portion 105 includes a second pixeltransistor 109, a third pixel transistor 123, a second liquid crystalelement 110, and a second capacitor 111.

In the light-transmitting pixel portion 104, a first terminal of thefirst pixel transistor 106 is connected to the first signal line 102 anda gate of the first pixel transistor 106 is connected to the scan line101. A first electrode (pixel electrode) of the first liquid crystalelement 107 is connected to a second terminal of the first pixeltransistor 106, and a second electrode (counter electrode) of the firstliquid crystal element 107 is connected to a common potential line 112(common line). A first electrode of the first capacitor 108 is connectedto the second terminal of the first pixel transistor 106, and a secondelectrode of the first capacitor 108 is connected to a capacitor line113.

In the reflective pixel portion 105, a first terminal of the secondpixel transistor 109 is connected to the second terminal of the firstpixel transistor 106 and a gate of the second pixel transistor 109 isconnected to the first selection line 103. A first terminal of the thirdpixel transistor 123 is connected to the second signal line 121, and agate of the third pixel transistor 123 is connected to the secondselection line 122. A first electrode (pixel electrode) of the secondliquid crystal element 110 is connected to a second terminal of thesecond pixel transistor 109 and a second terminal of the third pixeltransistor 123, and a second electrode (counter electrode) of the secondliquid crystal element 110 is connected to the common potential line112. A first electrode of the second capacitor 111 is connected to thesecond terminal of the second pixel transistor 109 and the secondterminal of the third pixel transistor 123, and a second electrode ofthe second capacitor 111 is connected to the capacitor line 113.

Note that each of the first pixel transistor 106, the second pixeltransistor 109, and the third pixel transistor 123 is preferably atransistor including an oxide semiconductor layer. The oxidesemiconductor is made to be intrinsic (i-type) by removal of hydrogenthat is an n-type impurity to be purified so that impurities that arenot main components of the oxide semiconductor are included as few aspossible. Note that a purified oxide semiconductor includes extremelyfew carriers (close to zero), and the carrier concentration thereof islower than 1×10¹⁴/cm³, preferably lower than 1×10¹²/cm³, more preferably1×10¹¹/cm³. Since the oxide semiconductor includes extremely fewcarriers, the off-state current of the transistor can be reduced.Specifically, in a transistor including the above oxide semiconductorlayer, the off-state current per micrometer in channel width at roomtemperature can be reduced to less than or equal to 10 aA/μm (1×10⁻¹⁷A/μm), further to less than or equal to 1 aA/μm (1×10⁻¹⁸ A/μm), stillfurther to less than or equal to 10 zA/μm (1×10⁻²⁰ A/μm). That is tosay, in circuit design, the oxide semiconductor can be regarded as aninsulator when the transistor is off. In the pixel 100 that is a pixelincluding the transistors which are formed using the oxide semiconductorand whose off-state current is extremely small, an image can bemaintained even when the number of times of writing of an image signal(also referred to as video voltage, a video signal, or video data) issmall and thus the refresh rate can be lowered. Therefore, a period inwhich a driver circuit for driving the scan line and the signal line isstopped can be provided and power consumption can be reduced.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor includes a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain may change depending on the structure, theoperating condition, and the like of the transistor, it is difficult todefine which is a source or a drain. Therefore, in this document (thespecification, the claims, the drawings, and the like), a regionfunctioning as a source and a drain is not called the source or thedrain in some cases. In such a case, for example, one of the source andthe drain may be referred to as a first terminal and the other thereofmay be referred to as a second terminal. Alternatively, one of thesource and the drain may be referred to as a first electrode and theother thereof may be referred to as a second electrode. Furtheralternatively, one of the source and the drain may be referred to as asource region and the other thereof may be referred to as a drainregion.

Note that when it is explicitly described that “A and B are connected,”the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein.

Note that voltage refers to a potential difference between a givenpotential and a reference potential (e.g., a ground potential) in manycases. Accordingly, voltage, a potential, and a potential difference canbe referred to as a potential, voltage, and a voltage difference,respectively.

Note that a common potential supplied to the common potential line 112may be any potential as long as it serves as a reference with respect toa potential of an image signal supplied to the first electrode of theliquid crystal element, and may be a ground potential, for example.

Note that an image signal may be appropriately inverted in accordancewith dot inversion driving, source line inversion driving, gate lineinversion driving, frame inversion driving, or the like to be input toeach pixel. Note also that the image signal is referred to by anothername such as a first image signal or a second image signal in somecases, depending on the kind of an image to be displayed.

Note that the potential of the capacitor line 113 may be the same as thecommon potential. Alternatively, another signal may be supplied to thecapacitor line 113.

Note that the second electrodes of the first liquid crystal element 107and the second liquid crystal element 110 are preferably provided tooverlap with the first electrodes of the first liquid crystal element107 and the second liquid crystal element 110. The first electrodes andthe second electrodes of the liquid crystal elements may each have ashape including a variety of opening patterns. As a liquid crystalmaterial provided between the first electrodes and the second electrodesin the liquid crystal elements, thermotropic liquid crystal,low-molecular liquid crystal, high-molecular liquid crystal, polymerdispersed liquid crystal, ferroelectric liquid crystal,anti-ferroelectric liquid crystal, or the like may be used. These liquidcrystal materials exhibit a cholesteric phase, a smectic phase, a cubicphase, a chiral nematic phase, an isotropic phase, or the like dependingon conditions. Alternatively, liquid crystal exhibiting a blue phase forwhich an alignment film is unnecessary may be used.

Note that the first electrode of the first liquid crystal element 107 inthe light-transmitting pixel portion 104 is formed using alight-transmitting material. As examples of the light-transmittingmaterial, indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide(IZO), zinc oxide to which gallium is added (GZO), and the like aregiven. On the other hand, a metal electrode with high reflectivity isused as the first electrode of the second liquid crystal element 110 inthe reflective pixel portion 105. Specifically, aluminum, silver, or thelike is used. In addition, outside light can be reflected irregularly bymaking a surface of the pixel electrode of the second liquid crystalelement 110 uneven. Note that the first electrode, the second electrode,and the liquid crystal material are collectively referred to as a liquidcrystal element in some cases.

Next, FIG. 2 is a schematic view of a liquid crystal display devicewhich includes the pixel 100 having the configuration illustrated inFIG. 1. In FIG. 2, a pixel portion 151, a scan line driver circuit 152(also referred to as a gate line driver circuit), a signal line drivercircuit 153 (also referred to as a data line driver circuit), and aterminal portion 154 are provided over a substrate 150.

Note that in FIG. 2, the scan line 101 is driven so that on/off of thefirst pixel transistor 106 is controlled by the scan line driver circuit152. An image signal to be supplied to the first liquid crystal element107 or the second liquid crystal element 110 is supplied to the firstsignal line 102 from the signal line driver circuit 153. An image signalto be supplied to the second liquid crystal element 110 is supplied tothe second signal line 121 from the terminal portion 154. A firstselection signal that controls on/off of the second pixel transistor 109is supplied to the first selection line 103 from the terminal portion154. A second selection signal that controls on/off of the third pixeltransistor 123 is supplied to the second selection line 122 from theterminal portion 154. Note that the scan line 101 is perpendicular tothe first selection line 103 and the second selection line 122 in FIG.2, but may be provided in parallel to the first selection line 103 andthe second selection line 122.

The scan line driver circuit 152 and the signal line driver circuit 153are preferably provided over the substrate over which the pixel portion151 is formed; however, these are not necessarily formed over thesubstrate over which the pixel portion 151 is formed. When the scan linedriver circuit 152 and the signal line driver circuit 153 are providedover the substrate over which the pixel portion 151 is formed, thenumber of the connection terminals for connection to the outside and thesize of the liquid crystal display device can be reduced.

Note that the plurality of pixels 100 is provided (arranged) in a matrixform in the pixel portion 151. Here, description that pixels areprovided (arranged) in a matrix form includes the case where the pixelsare provided in a straight line and the case where the pixels areprovided in a jagged line, in a longitudinal direction or a lateraldirection.

In addition to the first selection signal supplied to the firstselection line 103, signals for controlling the scan line driver circuit152 and the signal line driver circuit 153 (a high power supplypotential V_(dd), a low power supply potential V_(ss), a start pulse SP,and a clock signal CK, which are hereinafter referred to as drivercircuit control signals), fixed potentials supplied to the commonpotential line 112 and the capacitor line 113, the image signal suppliedto the second signal line 121, the second selection signal supplied tothe second selection line 122, and the like are supplied from theterminal portion 154. Note that the scan line driver circuit 152 and thesignal line driver circuit 153 to which the driver circuit controlsignals are supplied may each include a shift register circuit in whichflip-flop circuits or the like are cascaded. As for the first selectionsignal supplied to the first selection line 103 and the second selectionsignal supplied to the second selection line 122, the same signals maybe supplied all at once to the first selection line 103 and the secondselection line 122 which are connected to each pixel, which is differentfrom the case of a scan line or a signal line through which signals aresequentially supplied to a plurality of wirings. Note that the imagesignal supplied to the second signal line 121 is an image signal fordisplay at a black grayscale level in the reflective pixel portion 105;thus, the image signal having a potential for display at a blackgrayscale level may be supplied to the second liquid crystal element 110via the third pixel transistor 123, which is different from the case ofimage signals supplied to the scan line and the first signal line 102through which the image signals are sequentially supplied to a pluralityof wirings.

Next, operation of the liquid crystal display device will be describedwith reference to FIG. 3, FIGS. 4A and 4B, FIGS. 5A to 5E, and FIG. 6,in addition to FIG. 2.

As shown in FIG. 3, the operation of the liquid crystal display deviceis roughly divided into a moving image display period 301 and a stillimage display period 302. Note that the moving image display period 301and the still image display period 302 may be switched by supplying asignal for switching the periods from the outside or by judging themoving image display period 301 or the still image display period 302 onthe basis of an image signal.

The cycle of one frame period (or frame frequency) is preferably lessthan or equal to 1/60 sec (higher than or equal to 60 Hz) in the movingimage display period 301. The frame frequency is increased, so thatflickering is not sensed by a viewer of an image. In the still imagedisplay period 302, the cycle of one frame period is extremely long, forexample, longer than or equal to one minute (lower than or equal to0.017 Hz), so that eyestrain can be alleviated as compared to the casewhere the same image is switched plural times.

When an oxide semiconductor is used for semiconductor layers of thefirst pixel transistor 106, the second pixel transistor 109, and thethird pixel transistor 123, carriers in the oxide semiconductor can beextremely few as described above and thus the off-state current can bereduced. Accordingly, an electrical signal such as the image signal canbe held for a longer time in the pixel, and a writing interval can beset longer. Therefore, the cycle of one frame period can be set longer,and the frequency of refresh operation in the still image display period302 can be reduced, whereby an effect of suppressing power consumptioncan be further increased.

In the moving image display period 301 in FIG. 3, a color moving imagecan be displayed by field sequential driving, for example. Note thatcolor display may be performed using a color filter. In order to displaya moving image by field sequential driving, the driver circuit controlsignals are supplied to the scan line driver circuit 152 and the signalline driver circuit 153. In the moving image display period 301 in FIG.3, a backlight used for the color display by field sequential driving isoperated. Thus, a color moving image can be displayed on a displaypanel.

In the moving image display period 301, an image signal is supplied fromthe signal line driver circuit 153 through the first signal line 102 sothat color display (denoted by COLOR in the drawing) is performed in thelight-transmitting pixel portion 104, and an image signal is suppliedfrom the terminal portion 154 through the second signal line 121 so thatdisplay at a black grayscale level (denoted by BK in the drawing) isperformed in the reflective pixel portion 105. Thus, the contrast in thelight-transmitting pixel portion 104, which is reduced by lightscattering caused by irradiation of outside light on the reflectivepixel portion 105, can be recovered.

In the still image display period 302 in FIG. 3, an image signal issupplied through the first signal line 102 so that a black-and-whitegrayscale (denoted by BK/W in the drawing) is displayed depending onwhether reflected light is transmitted or not, whereby a still image canbe displayed. In the still image display period 302, the driver circuitcontrol signals are supplied only when the black-and-white grayscaleimage signals are written, and supply of the driver circuit controlsignals is partly or completely stopped in a period in which the imagesignal which has been written is held, that is, in a period except theperiod in which the black-and-white grayscale image signal is written.Therefore, power consumption can be reduced in the still image displayperiod 302 owing to the period in which the supply of the driver circuitcontrol signals is stopped. Moreover, display comes to be visible byutilizing reflected light of outside light in the still image displayperiod 302 in FIG. 3; therefore, the backlight is not operated. Thus, ablack-and-white grayscale still image can be displayed on the displaypanel.

As for the stop of the supply of the driver circuit control signals, inthe case where the holding period of the image signal which has beenwritten is short, a configuration in which supply of the high powersupply potential V_(dd) and the low power supply potential V_(ss) is notstopped may be originally employed. This is because increase in powerconsumption due to repetition of stop and start of supply of the highpower supply potential V_(dd) and the low power supply potential V_(ss)can be reduced, which is favorable.

Next, the moving image display period 301 and the still image displayperiod 302 of FIG. 3 will be described in detail with reference totiming charts of FIGS. 4A and 4B, respectively. The timing charts ofFIGS. 4A and 4B are exaggerated for description.

First, FIG. 4A will be described. FIG. 4A shows the driver circuitcontrol signals supplied to the scan line driver circuit 152 and thesignal line driver circuit 153, image signals, and the state of thebacklights in one frame period of the moving image display period 301,as an example. As for the backlights, the case where lights emittingthree colors of red (R), green (G), and blue (B) are sequentially turnedon is described. By using LEDs as the backlights, lower powerconsumption and longer lifetime can be achieved.

In the moving image display period 301, a moving image is displayed byfield sequential driving; therefore, operation in the light-transmittingpixel portion 104 is performed in such a manner that an image signal forred (R) display is written into each pixel through the first signal line102 first, a backlight of R is then turned on, an image signal for green(G) display is written into each pixel through the first signal line 102next, a backlight of G is then turned on, an image signal for blue (B)display is written into each pixel through the first signal line 102next, and a backlight of B is then turned on. Next, in the moving imagedisplay period 301, after the image signals of R, G, and B are writtenand the backlights of R, G, and B are turned on, operation is performedso that an image signal for display at a black grayscale level issupplied to the reflective pixel portion 105 through the second signalline 121. Further, in the moving image display period 301, the drivercircuit control signals are supplied to driver circuits, so that boththe scan line driver circuit 152 and the signal line driver circuit 153are operated.

In short, the moving image display period 301 can be roughly dividedinto an image signal writing period (T1 in FIG. 4A, which is alsoreferred to as a first period), a backlight lighting period (T2 in FIG.4A, which is also referred to as a second period), and a black grayscalesignal writing period (T3 in FIG. 4A, which is also referred to as athird period).

By repeating the above operation so that image signals are changed, aviewer can perceive color display of a moving image. Note that the orderof R, G, and B in FIG. 4A may be a different order or display may beperformed using more colors. The black grayscale signal writing period,which is the third period T3, is provided once in one frame period inFIG. 4A, but may be provided once in a plurality of frame periods.

Note that the supply of the image signal for display at a blackgrayscale level to the reflective pixel portion 105 may be performedbefore the image signal writing period and the backlight lightingperiod. Thus, the contrast in the light-transmitting pixel portion 104,which is reduced by light scattering caused by irradiation of outsidelight on the reflective pixel portion 105, can be recovered.

Next, FIG. 4B will be described. Similarly to FIG. 4A, FIG. 4B shows thedriver circuit control signals supplied to the scan line driver circuit152 and the signal line driver circuit 153, an image signal, and thestate of the backlights in one frame period of the still image displayperiod 302.

In the still image display period 302, an image signal for displaying ablack-and-white grayscale image depending on whether reflected light istransmitted or not is supplied through the first signal line 102. Atthis time, the backlights are not operated, and the driver circuitcontrol signals are supplied to driver circuits, so that both the scanline driver circuit 152 and the signal line driver circuit 153 areoperated. Next, the image signal for displaying a black-and-whitegrayscale image which has been written is held, so that a still image isdisplayed. At this time, an additional image signal is not written, thebacklights are not operated, and the driver circuit control signals arenot supplied. Therefore, power consumed by the backlights and the drivercircuit control signals can be reduced; thus, lower power consumptioncan be achieved. As for the holding of the still image, the image signalwritten into a pixel is held by a pixel transistor whose off-statecurrent is extremely small; therefore, the black-and-white grayscalestill image can be held for longer than or equal to one minute. Inaddition, the still image may be held in the following manner: beforethe level of the image signal held is lowered after a certain period oftime, a new still image signal which is the same image signal as thestill image signal of the previous period is written (refresh operation)and the still image is held again.

The still image display period 302 can be roughly divided into a stillimage signal writing period (T4 in FIG. 4B, which is also referred to asa fourth period) and a still image signal holding period (T5 in FIG. 4B,which is also referred to as a fifth period).

Next, how the pixel 100 in FIG. 1 is operated in the periods T1 to T5 inFIGS. 4A and 4B will be described with reference to FIGS. 5A to 5E whichillustrate signals and on/off of pixel transistors. Although not allcomponents are denoted by reference numerals in FIGS. 5A to 5E,description is given using the same reference numerals as FIG. 1.Further, dotted arrows in FIGS. 5A to 5E are shown to facilitateunderstanding of signal flow. “ON” and “OFF” in FIGS. 5A to 5E representon and off of the pixel transistors, respectively. “COLOR” in FIGS. 5Ato 5E is shown to facilitate understanding of the state where a colorimage signal (a first image signal) is supplied to or held in the signalline, the light-transmitting pixel portion, or the reflective pixelportion. Similarly, “BK” represents a black grayscale image signal (animage signal for black display), and “BK/W” represents a black-and-whiteimage signal (a second image signal).

First, in the period T1 illustrated in FIG. 5A, that is, in the imagesignal writing period, the scan line 101 is controlled so that the firstpixel transistor 106 is turned on, the first image signal (denoted byCOLOR in the drawing) is supplied to the first signal line 102, and thefirst image signal is written into the first liquid crystal element 107in the light-transmitting pixel portion 104; thus, the alignment ofliquid crystal in the light-transmitting pixel portion 104 iscontrolled. At this time, in the reflective pixel portion 105, the firstselection line 103 is controlled so that the second pixel transistor 109is turned off, the second selection line 122 is controlled so that thethird pixel transistor 123 is turned off, the first image signal of thefirst signal line 102 and the black grayscale image signal of the secondsignal line 121 are not written into the second liquid crystal element110, and a black grayscale image signal (denoted by BK in the drawing)which has been written in the previous frame period is held; thus, thesecond liquid crystal element 110 in the reflective pixel portion 105 iscontrolled.

Note that in the image signal writing period, a configuration may beemployed in which the scan line 101 is controlled so that the firstpixel transistor 106 is turned on, the first image signal is supplied tothe first signal line 102 to be written into the first liquid crystalelement 107, the second selection line 122 is controlled so that thethird pixel transistor 123 is turned on, and the black grayscale imagesignal of the second signal line 121 is written into the second liquidcrystal element 110. This operation is illustrated in FIG. 6. The on/offof the third pixel transistor 123 is controlled by the second selectionline 122, whereby the black grayscale image signal can be suppliedthrough the second signal line 121 separately from the first signal line102. Accordingly, the black grayscale signal writing period describedlater can be omitted and thus one frame period can be shortened. Afterthe black grayscale image signal is written into the second liquidcrystal element 110, the third pixel transistor 123 may be turned off.

Note that the writing of the signal for black display into the secondliquid crystal element 110 in FIG. 6 may be performed all at once in allthe pixels. The writing of the signal for black display into the secondliquid crystal element 110 can be performed separately from the firstsignal line 102, and thus can be performed at any time. Note that it ispreferable that the black grayscale image signal be written into thesecond liquid crystal element 110 in advance in the backlight lightingperiod. This is for prevention of reduction in contrast.

Then, in the period T2 illustrated in FIG. 5B, that is, in the backlightlighting period, the scan line 101 is controlled so that the first pixeltransistor 106 is turned off, and light from the backlight istransmitted or not transmitted depending on the alignment of liquidcrystal corresponding to the first image signal (denoted by COLOR in thedrawing) which has been written in the image signal writing period inthe light-transmitting pixel portion 104. At this time, in thereflective pixel portion 105, the first selection line 103 is controlledso that the second pixel transistor 109 is turned off, the secondselection line 122 is controlled so that the third pixel transistor 123is turned off, and the black grayscale image signal (denoted by BK inthe drawing) which has been written in the previous frame period isheld; thus, the second liquid crystal element 110 in the reflectivepixel portion 105 is controlled.

Next, in the period T3 illustrated in FIG. 5C, that is, in the blackgrayscale signal writing period, the scan line 101 is controlled so thatthe first pixel transistor 106 is turned off and an image signal is notwritten into the first liquid crystal element 107 in thelight-transmitting pixel portion 104. At this time, in the reflectivepixel portion 105, the first selection line 103 is controlled so thatthe second pixel transistor 109 is turned off, the second selection line122 is controlled so that the third pixel transistor 123 is turned on,and the black grayscale image signal of the second signal line 121 iswritten into the second liquid crystal element 110; thus, the secondliquid crystal element 110 in the reflective pixel portion 105 iscontrolled. Note that after the black grayscale image signal is writteninto the second liquid crystal element 110, the third pixel transistor123 may be turned off.

Note that the writing of the signal for black display into the secondliquid crystal element 110 in the period T3 may be performed all at oncein all the pixels. By writing the signal for black display all at onceor by writing the signal for black display by line sequential driving,the period T3 can be shortened and the image quality can be improved.

Next, in the period T4 illustrated in FIG. 5D, that is, in the stillimage signal writing period, the scan line 101 is controlled so that thefirst pixel transistor 106 is turned on, the second image signal(denoted by BK/W in the drawing) is supplied to the first signal line102, and the signal for black-and-white display is written into thefirst liquid crystal element 107 in the light-transmitting pixel portion104; thus, the alignment of liquid crystal in the light-transmittingpixel portion 104 is controlled. At this time, in the reflective pixelportion 105, the first selection line 103 is controlled so that thesecond pixel transistor 109 is turned on, the second selection line 122is controlled so that the third pixel transistor 123 is turned off, andthe second image signal of the first signal line 102 is written into thesecond liquid crystal element 110; thus, the second liquid crystalelement 110 in the reflective pixel portion 105 is controlled.

Then, in the period T5 illustrated in FIG. 5E, that is, in the stillimage signal holding period, the scan line 101 is controlled so that thefirst pixel transistor 106 is turned off and the alignment of liquidcrystal is controlled in response to the second image signal (denoted byBK/W in the drawing) which has been written in the still image signalwriting period in the light-transmitting pixel portion 104. At thistime, in the reflective pixel portion 105, the first selection line 103is controlled so that the second pixel transistor 109 is turned off, thesecond selection line 122 is controlled so that the third pixeltransistor 123 is turned off, and the second image signal (denoted byBK/W in the drawing) which has been written in the still image signalwriting period is held; thus, the second liquid crystal element 110 inthe reflective pixel portion 105 is controlled.

Note that in the still image signal writing period illustrated in FIG.5D, the second image signal is written into the reflective pixel portion105 and the second image signal is also written into thelight-transmitting pixel portion 104. Although the backlight is notoperated in the still image signal holding period illustrated in FIG.5E, an image might be dark and difficult to see owing to insufficientreflection of light in the reflective pixel portion 105, depending onthe environment or the like. In such a case, the visibility can besecured by operating the backlight and switching display of thereflective pixel portion 105 to the display of the light-transmittingpixel portion 104 into which the second image signal at the samegrayscale level has been written. The switching of an operating stateand a non-operating state of the backlight may be performed only whenthe visibility is insufficient; therefore, an optical sensor or the likemay be additionally provided and the switching may be performed inaccordance with the illuminance of the environment. Note that theoperating state and the non-operating state of the backlight may beswitched by manual operation with a switch or the like. Further, byusing an oxide semiconductor for the first pixel transistor 106, thesecond pixel transistor 109, and the third pixel transistor 123, theoff-state current thereof can be reduced. Reduction in off-state currentleads to a long still image signal holding period; therefore, the use ofan oxide semiconductor is preferable for reduction in power consumption.

In the still image signal holding period, the frequency of operationsuch as writing of an image signal can be reduced. When seeing an imageformed by writing image signals a plurality of times, the human eyesrecognize images switched a plurality of times, which might lead toeyestrain. With a structure in which the frequency of writing of imagesignals is reduced as described in this embodiment, eyestrain can bealleviated.

In the above-described manner, according to an embodiment of the presentinvention, reduction in contrast due to light scattering or the like ina reflective pixel portion can be suppressed and power consumption canbe reduced without making the structure complicated, for example,increase in the number of driver circuits, wirings, and the like.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments as appropriate.

Embodiment 2

In this embodiment, structures of a top view and a cross-sectional viewcorresponding to the circuit diagram of the pixel of the liquid crystaldisplay device illustrated in FIG. 1 of Embodiment 1 will be described.

FIGS. 7A and 7B are a top view and a cross-sectional view, respectively,in the case where inverted staggered transistors are used as the firstpixel transistor 106, the second pixel transistor 109, and the thirdpixel transistor 123 described in Embodiment 1. The cross-sectional viewof a pixel transistor illustrated in FIG. 7B corresponds to a crosssection along line A-A′ in the top view of the pixel illustrated in FIG.7A.

First, an example of a layout of a pixel of a liquid crystal displaydevice will be described with reference to FIG. 7A. Note that FIGS. 7Aand 7B illustrate a structure applied to the pixel 100 in FIG. 1described in Embodiment 1.

The pixel in FIG. 7A that can be applied to the liquid crystal displaydevice of Embodiment 1 includes a scan line 801, a first signal line802, a first selection line 803, a capacitor line 804, a secondselection line 805, a second signal line 806, a first pixel transistor807, a first pixel electrode 808, a first capacitor 809, a second pixeltransistor 810, a second pixel electrode 811, a second capacitor 812,and a third pixel transistor 813 as components corresponding to those inFIG. 1. The components are formed using a conductive layer 851, asemiconductor layer 852, a conductive layer 853, a transparentconductive layer 854, a reflective conductive layer 855, a contact hole856, and a contact hole 857.

The conductive layer 851 has a region that functions as a gate electrodeor a scan line. The semiconductor layer 852 has regions that function assemiconductor layers of the pixel transistors. The conductive layer 853has regions that function as a wiring and sources and drains of thepixel transistors. The transparent conductive layer 854 has a regionthat functions as a pixel electrode of a first liquid crystal element.The reflective conductive layer 855 has a region that functions as apixel electrode of a second liquid crystal element. The conductive layer851 and the conductive layer 853 are connected to each other through thecontact hole 856. The conductive layer 853 and the conductive layer 854or the conductive layer 853 and the conductive layer 855 are connectedto each other through the contact hole 857.

Note that FIG. 8 illustrates a layout of a pixel in which the reflectiveconductive layer 855 is not shown. As illustrated in FIG. 8, the secondpixel transistor 810 and the second capacitor 812 are provided tooverlap with the reflective conductive layer 855. The second capacitor812 is provided in a position overlapping with the reflective conductivelayer 855; thus, the capacitance can be increased without reduction inthe aperture ratio.

In order to reflect incident outside light irregularly, the reflectiveconductive layer 855 is preferably subjected to treatment for making asurface thereof uneven.

In the layouts of the pixels in FIG. 7A and FIG. 8, the first pixelelectrode 808 and the first signal line 802 are provided to be apartfrom each other. By providing the first pixel electrode 808 and thefirst signal line 802 to be apart from each other, variation in thepotential of the first pixel electrode 808 due to variation in thepotential of the signal line can be reduced.

In the layouts of the pixels in FIG. 7A and FIG. 8, the conductive layer851 is preferably provided so as to surround the first pixel electrode808. With the structure in which the first pixel electrode 808 issurrounded by the conductive layer 851, a light-blocking portion (blackmatrix) which is provided so as to surround the first pixel electrode808 can be omitted. Moreover, it is preferable that the conductive layer851 be provided between the transparent conductive layer 854 and thereflective conductive layer 855 because a difference in height betweensurfaces of the transparent conductive layer 854 and the reflectiveconductive layer 855 can be reduced.

In the layouts of the pixels in FIG. 7A and FIG. 8, the first selectionline 803 and the capacitor line 804 are provided in parallel to thefirst signal line 802. By providing the first selection line 803, thecapacitor line 804, and the first signal line 802 in parallel to oneanother, the capacitance in an intersection between the wirings can bereduced. Accordingly, noise, delay of a signal, distortion of a signalwaveform, or the like can be reduced.

Next, the structure of the cross-sectional view of FIG. 7B will bedescribed. In this embodiment, a method for forming a transistorparticularly when a semiconductor layer is formed using an oxidesemiconductor will be described. FIG. 7B illustrates a transistorincluding an oxide semiconductor as a semiconductor layer. An advantageof using an oxide semiconductor is that high mobility and low off-statecurrent can be obtained in relatively easy and low-temperatureprocesses; needless to say, another semiconductor may be used.

A transistor 410 illustrated in FIG. 7B is a kind of bottom-gatetransistors, and is also referred to as an inverted staggeredtransistor. Note that there is no particular limitation on a structureof a transistor which can be applied to a liquid crystal display devicedisclosed in this specification. For example, a top-gate staggeredstructure, a bottom-gate staggered structure, a top-gate planarstructure, a bottom-gate planar structure, or the like can be used. Thetransistor may have a single-gate structure in which one channelformation region is formed, a double-gate structure in which two channelformation regions are formed, or a triple-gate structure in which threechannel formation regions are formed. Alternatively, the transistor mayhave a dual-gate structure including two gate electrode layerspositioned above and below a channel region with gate insulating layersprovided therebetween.

The transistor 410 includes, over a substrate 400 having an insulatingsurface, a gate electrode layer 401, a gate insulating layer 402, anoxide semiconductor layer 403, a source electrode layer 405 a, and adrain electrode layer 405 b. In addition, an insulating layer 407 whichcovers the transistor 410 and is stacked over the oxide semiconductorlayer 403 is provided. A protective insulating layer 409 is formed overthe insulating layer 407.

In this embodiment, as described above, the oxide semiconductor layer403 is used as a semiconductor layer. As an oxide semiconductor used forthe oxide semiconductor layer 403, an In—Sn—Ga—Zn—O-based metal oxidewhich is a four-component metal oxide; an In—Ga—Zn—O-based metal oxide,an In—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metal oxide, aSn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, or aSn—Al—Zn—O-based metal oxide which is a three-component metal oxide; anIn—Zn—O-based metal oxide, a Sn—Zn—O-based metal oxide, an Al—Zn—O-basedmetal oxide, a Zn—Mg—O-based metal oxide, a Sn—Mg—O-based metal oxide,or an In—Mg—O-based metal oxide which is a two-component metal oxide; anIn—O-based metal oxide, a Sn—O-based metal oxide, a Zn—O-based metaloxide, or the like can be used. Further, SiO₂ may be included in asemiconductor of the above metal oxide. Here, for example, anIn—Ga—Zn—O-based metal oxide is an oxide including at least In, Ga, andZn, and there is no particular limitation on the composition ratiothereof. Further, the In—Ga—Zn—O-based metal oxide may include anelement other than In, Ga, and Zn.

For the oxide semiconductor layer 403, a thin film represented by thechemical formula, InMO₃(ZnO), (m>0) can be used. Here, M represents oneor more metal elements selected from Ga, Al, Mn, and Co. For example, Mcan be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In the transistor 410 including the oxide semiconductor layer 403, acurrent value in an off state (off-state current value) can be reduced.Therefore, an electrical signal of image data or the like can be heldfor a longer time, so that a writing interval can be set longer.Accordingly, the frequency of refresh operation can be reduced, whichleads to an effect of suppressing power consumption.

Although there is no particular limitation on a substrate that can beused as the substrate 400 having an insulating surface, a glasssubstrate of barium borosilicate glass, aluminoborosilicate glass, orthe like is used.

In the bottom-gate transistor 410, an insulating layer serving as a basefilm may be provided between the substrate and the gate electrode layer.The base film has a function of preventing diffusion of an impurityelement from the substrate, and can be formed to have a single-layerstructure or a stacked-layer structure using one or more layers selectedfrom a silicon nitride layer, a silicon oxide layer, a silicon nitrideoxide layer, and a silicon oxynitride layer.

The gate electrode layer 401 can be formed to have a single-layerstructure or a stacked-layer structure using a metal material such asmolybdenum (Mo), titanium (Ti), chromium (Cr), tantalum (Ta), tungsten(W), aluminum (Al), copper (Cu), neodymium (Nd), or scandium (Sc), or analloy material including any of these as a main component.

The gate insulating layer 402 can be formed to have a single-layerstructure or a stacked-layer structure using any of a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, a siliconnitride oxide layer, an aluminum oxide layer, an aluminum nitride layer,an aluminum oxynitride layer, an aluminum nitride oxide layer, and ahafnium oxide layer by a plasma CVD method, a sputtering method, or thelike. For example, a silicon nitride layer (SiN_(y) (y>0)) with athickness of greater than or equal to 50 nm and less than or equal to200 nm is formed by a plasma CVD method as a first gate insulatinglayer, and a silicon oxide layer (SiO_(x) (x>0)) with a thickness ofgreater than or equal to 5 nm and less than or equal to 300 nm is formedas a second gate insulating layer over the first gate insulating layer,so that a gate insulating layer with a thickness of 200 nm in total isformed.

A conductive film used for the source electrode layer 405 a and thedrain electrode layer 405 b can be formed using an element selected fromAl, Cr, Cu, Ta, Ti, Mo, and W, an alloy including any of these elementsas a component, an alloy film including a combination of any of theseelements, or the like. Alternatively, a structure may be employed inwhich a high-melting-point metal layer of Ti, Mo, W, or the like isstacked on one or both of a top surface and a bottom surface of a metallayer of Al, Cu, or the like. In addition, heat resistance can beimproved by using an Al material to which an element (such as Si, Nd, orSc) which prevents generation of a hillock or a whisker in an Al film isadded.

Alternatively, the conductive film to be the source electrode layer 405a and the drain electrode layer 405 b (including a wiring layer formedusing the same layer as the source electrode layer 405 a and the drainelectrode layer 405 b) may be formed using a conductive metal oxide. Asthe conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), an indium oxide-tin oxide alloy (In₂O₃—SnO₂, which isabbreviated to ITO), an indium oxide-zinc oxide alloy (In₂O₃—ZnO), orany of these metal oxide materials including silicon oxide can be used.

As the insulating layer 407, typically, an inorganic insulating filmsuch as a silicon oxide film, a silicon oxynitride film, an aluminumoxide film, or an aluminum oxynitride film can be used.

As the protective insulating layer 409, an inorganic insulating filmsuch as a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, or an aluminum nitride oxide film can be used.

In addition, a planarization insulating film may be formed over theprotective insulating layer 409 in order to reduce surface unevennessdue to the transistor. As the planarization insulating film, an organicmaterial such as polyimide, acrylic, or benzocyclobutene can be used.Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material) or the like. Notethat the planarization insulating film may be formed by stacking aplurality of insulating films formed using any of these materials. Notethat a needed component such as a reflective conductive layer or aliquid crystal layer may be provided as appropriate over the protectiveinsulating layer 409.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, an example of an electronic device including theliquid crystal display device described in any of the above embodimentswill be described.

FIG. 9A illustrates an electronic book reader (also referred to as ane-book reader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar cell 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader illustrated in FIG. 9Acan have a function of displaying various kinds of information (such asa still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, time, or the like on the display portion,a function of operating or editing the information displayed on thedisplay portion, a function of controlling processing by various kindsof software (programs), and the like. Note that in FIG. 9A, a battery9635 and a DCDC converter 9636 (hereinafter abbreviated to a converter)are included in the charge and discharge control circuit 9634, as anexample.

When a semi-transmissive liquid crystal display device is used as thedisplay portion 9631, the electronic book reader is expected to be usedin a relatively bright environment, in which case the structureillustrated in FIG. 9A is preferable because power generation by thesolar cell 9633 and charge in the battery 9635 can be efficientlyperformed. Note that the solar cell 9633 can be configured so that thebattery 9635 is charged on a front surface and a rear surface of thehousing 9630, which is preferable. When a lithium ion battery is used asthe battery 9635, there is an advantage of downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 9A will be described with reference toa block diagram in FIG. 9B. The solar cell 9633, the battery 9635, theconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are illustrated in FIG. 9B, and the battery 9635, theconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634.

First, an example of operation in the case where power is generated bythe solar cell 9633 with the use of outside light is described. Thevoltage of power generated by the solar cell 9633 is raised or loweredby the converter 9636 so as to be voltage for charging the battery 9635.Then, when the power from the solar cell 9633 is used for the operationof the display portion 9631, the switch SW1 is turned on and the voltageof the power is raised or lowered by the converter 9637 to be voltageneeded for the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Next, an example of operation in the case where power is not generatedby the solar cell 9633 with the use of outside light is described. Thevoltage of power accumulated in the battery 9635 is raised or lowered bythe converter 9637 by turning on the switch SW3. Then, power from thebattery 9635 is used for the operation of the display portion 9631.

Note that although the solar cell 9633 is described as an example of ameans for charge, charge of the battery 9635 may be performed withanother means. In addition, a combination of the solar cell 9633 andanother means for charge may be used.

This embodiment can be implemented in combination with any of thestructures described in the other embodiments as appropriate.

This application is based on Japanese Patent Application serial no.2010-009681 filed with Japan Patent Office on Jan. 20, 2010, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

100: pixel, 101: scan line, 102: first signal line, 103: first selectionline, 104: light-transmitting pixel portion, 105: reflective pixelportion, 106: first pixel transistor, 107: first liquid crystal element,108: first capacitor, 109: second pixel transistor, 110: second liquidcrystal element, 111: second capacitor, 112: common potential line, 113:capacitor line, 121: second signal line, 122: second selection line,123: third pixel transistor, 150: substrate, 151: pixel portion, 152:scan line driver circuit, 153: signal line driver circuit, 154: terminalportion, 301: moving image display period, 302: still image displayperiod, 400: substrate, 401: gate electrode layer, 402: gate insulatinglayer, 403: oxide semiconductor layer, 405a: source electrode layer,405b: drain electrode layer, 407: insulating layer, 409: protectiveinsulating layer, 410: transistor, 801: scan line, 802: first signalline, 803: first selection line, 804: capacitor line, 805: secondselection line, 806: second signal line, 807: first pixel transistor,808: first pixel electrode, 809: first capacitor, 810: second pixeltransistor, 811: second pixel electrode, 812: second capacitor, 813:third pixel transistor, 851: conductive layer, 852: semiconductor layer,853: conductive layer, 854: transparent conductive layer, 855:reflective conductive layer, 856: contact hole, 857: contact hole, 9630:housing, 9631: display portion, 9632: operation key, 9633: solar cell,9634: charge and discharge control circuit, 9635: battery, 9636:converter, and 9637: converter.

The invention claimed is:
 1. A liquid crystal display device comprising:a pixel having a light-transmitting pixel portion and a reflective pixelportion, the light-transmitting pixel portion comprising: a first pixeltransistor whose first terminal is electrically connected to a firstsignal line and whose gate is electrically connected to a scan line; anda first liquid crystal element and a first capacitor which areelectrically connected to a second terminal of the first pixeltransistor, the reflective pixel portion comprising: a second pixeltransistor whose first terminal is electrically connected to the secondterminal of the first pixel transistor and whose gate is electricallyconnected to a first selection line; a second liquid crystal element anda second capacitor which are electrically connected to a second terminalof the second pixel transistor; and a third pixel transistor whose firstterminal is electrically connected to a second signal line, whose gateis electrically connected to a second selection line, and whose secondterminal is electrically connected to the second liquid crystal elementand the second capacitor.
 2. The liquid crystal display device,according to claim 1, further comprising: an optical sensor whichcontrols a backlight.
 3. The liquid crystal display device, according toclaim 1, wherein at least one of the first pixel transistor, the secondpixel transistor, and the third pixel transistor includes an oxidesemiconductor layer.
 4. The liquid crystal display device, according toclaim 1, wherein a carrier concentration of at least one of the firstpixel transistor, the second pixel transistor, and the third pixeltransistor is lower than 1×10¹⁴/cm³.
 5. The liquid crystal displaydevice, according to claim 1, wherein an off-state current of at leastone of the first pixel transistor, the second pixel transistor, and thethird pixel transistor is less than or equal to 1×10⁻¹⁷ A/μm.
 6. Theliquid crystal display device, according to claim 1, wherein a part ofthe scan line and the first capacitor are provided so as to surround anelectrode of the first liquid crystal element.
 7. The liquid crystaldisplay device, according to claim 1, wherein the second pixeltransistor and the second capacitor are provided to overlap with anelectrode of the second liquid crystal element.
 8. The liquid crystaldisplay device, according to claim 1, wherein the first terminal of thethird pixel transistor is directly connected to the second signal line,and wherein the second terminal of the third pixel transistor isdirectly connected to the second liquid crystal element and the secondcapacitor.
 9. The liquid crystal display device, according to claim 1,wherein the first liquid crystal element and the first capacitor aredirectly connected to the second terminal of the first pixel transistor,and wherein the first terminal of the second pixel transistor isdirectly connected to the second terminal of the first pixel transistor.10. A method for driving a liquid crystal display device comprising aplurality of pixels, wherein each of the pixels comprises alight-transmitting pixel portion and a reflective pixel portion, whereinthe light-transmitting pixel portion comprises a first pixel transistorwhose first terminal is electrically connected to a first signal lineand whose gate is electrically connected to a scan line, and a firstliquid crystal element and a first capacitor which are electricallyconnected to a second terminal of the first pixel transistor, andwherein the reflective pixel portion comprises a second pixel transistorwhose first terminal is electrically connected to the second terminal ofthe first pixel transistor and whose gate is electrically connected to afirst selection line, a second liquid crystal element and a secondcapacitor which are electrically connected to a second terminal of thesecond pixel transistor, and a third pixel transistor whose firstterminal is electrically connected to a second signal line, whose gateis electrically connected to a second selection line, and whose secondterminal is electrically connected to the second liquid crystal elementand the second capacitor, the method for driving the liquid crystaldisplay device, comprising the steps of: in a first period, turning onthe first pixel transistor, turning off the second pixel transistor, andsupplying a first image signal to the first liquid crystal element andthe first capacitor from the first signal line; in a second period,performing display in the light-transmitting pixel portion in responseto the first image signal supplied in the first period; in a thirdperiod, turning on the third pixel transistor, and supplying a signalfor black display to the second liquid crystal element and the secondcapacitor from the second signal line; repeating the first to thirdperiods so that a moving image is displayed; in a fourth period, turningon the first pixel transistor, turning on the second pixel transistor,turning off the third pixel transistor, and supplying a second imagesignal to the first liquid crystal element, the first capacitor, thesecond liquid crystal element, and the second capacitor from the firstsignal line; in a fifth period, performing display in the reflectivepixel portion in response to the second image signal supplied in thefourth period; and repeating the fourth and fifth periods so that astill image is displayed.
 11. The method for driving a liquid crystaldisplay device, according to claim 10, wherein the first image signal isan image signal corresponding to any color of R, G, and B, and wherein abacklight which emits the corresponding color of R, G, and B issequentially operated in the second period.
 12. The method for driving aliquid crystal display device, according to claim 10, wherein the secondimage signal is an image signal for displaying an image at a lowergrayscale level than an image of the first image signal.
 13. The methodfor driving a liquid crystal display device, according to claim 10,wherein time for displaying one image in the fourth and fifth periods islonger than time for displaying one image in the first to third periods.14. The method for driving a liquid crystal display device, according toclaim 10, wherein supply of a driver circuit control signal for drivingthe scan line and the first signal line is stopped in the fifth period.15. The method for driving a liquid crystal display device, according toclaim 10, further comprising the step of: switching between a first modeincluding the first to third periods and a second mode including thefourth and fifth periods.
 16. The method for driving a liquid crystaldisplay device, according to claim 10, further comprising the step of:switching between a first mode including the first to third periods anda second mode including the fourth and fifth periods on the basis of thefirst or second image signal.
 17. The method for driving a liquidcrystal display device, according to claim 10, wherein a frame frequencyis higher than or equal to 60 Hz in the first to third periods.
 18. Themethod for driving a liquid crystal display device, according to claim10, wherein a frame frequency is lower than or equal to 0.017 Hz in thefourth and fifth periods.
 19. The method for driving a liquid crystaldisplay device, according to claim 10, further comprising the step of:controlling a backlight by an optical sensor in the fifth period.
 20. Amethod for driving a liquid crystal display device comprising aplurality of pixels, wherein each of the pixels comprises alight-transmitting pixel portion and a reflective pixel portion, whereinthe light-transmitting pixel portion comprises a first pixel transistorwhose first terminal is electrically connected to a first signal lineand whose gate is electrically connected to a scan line, and a firstliquid crystal element and a first capacitor which are electricallyconnected to a second terminal of the first pixel transistor, andwherein the reflective pixel portion comprises a second pixel transistorwhose first terminal is electrically connected to the second terminal ofthe first pixel transistor and whose gate is electrically connected to afirst selection line, a second liquid crystal element and a secondcapacitor which are electrically connected to a second terminal of thesecond pixel transistor, and a third pixel transistor whose firstterminal is electrically connected to a second signal line, whose gateis electrically connected to a second selection line, and whose secondterminal is electrically connected to the second liquid crystal elementand the second capacitor, the method for driving the liquid crystaldisplay device, comprising the steps of: in a first period, turning onthe first pixel transistor, turning off the second pixel transistor,turning off the third pixel transistor, and supplying a first imagesignal to the first liquid crystal element and the first capacitor fromthe first signal line; in a second period, performing display in thelight-transmitting pixel portion in response to the first image signalsupplied in the first period; in a third period, turning off the firstpixel transistor, turning off the second pixel transistor, turning onthe third pixel transistor, and supplying a signal for black display tothe second liquid crystal element and the second capacitor from thesecond signal line in the reflective pixel portion; repeating the firstto third periods so that a moving image is displayed; in a fourthperiod, turning on the first pixel transistor, turning on the secondpixel transistor, turning off the third pixel transistor, and supplyinga second image signal to the first liquid crystal element, the firstcapacitor, the second liquid crystal element, and the second capacitorfrom the first signal line; in a fifth period, performing display in thereflective pixel portion in response to the second image signal suppliedin the fourth period; and repeating the fourth period and the fifthperiod so that a still image is displayed.
 21. The method for driving aliquid crystal display device, according to claim 20, wherein the firstimage signal is an image signal corresponding to any color of R, G, andB, and wherein a backlight which emits the corresponding color of R, G,and B is sequentially operated in the second period.
 22. The method fordriving a liquid crystal display device, according to claim 20, whereinthe second image signal is an image signal for displaying an image at alower grayscale level than an image of the first image signal.
 23. Themethod for driving a liquid crystal display device, according to claim20, wherein time for displaying one image in the fourth and fifthperiods is longer than time for displaying one image in the first tothird periods.
 24. The method for driving a liquid crystal displaydevice, according to claim 20, wherein supply of a driver circuitcontrol signal for driving the scan line and the first signal line isstopped in the fifth period.
 25. The method for driving a liquid crystaldisplay device, according to claim 20, further comprising the step of:switching between a first mode including the first to third periods anda second mode including the fourth and fifth periods.
 26. The method fordriving a liquid crystal display device, according to claim 20, furthercomprising the step of: switching between a first mode including thefirst to third periods and a second mode including the fourth and fifthperiods on the basis of the first or second image signal.
 27. The methodfor driving a liquid crystal display device, according to claim 20,wherein a frame frequency is higher than or equal to 60 Hz in the firstto third periods.
 28. The method for driving a liquid crystal displaydevice, according to claim 20, wherein a frame frequency is lower thanor equal to 0.017 Hz in the fourth and fifth periods.
 29. The method fordriving a liquid crystal display device, according to claim 20, furthercomprising the step of: controlling a backlight by an optical sensor inthe fifth period.